Repurchase Agreements (Details) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Repurchase Agreements
Balance of repurchase agreements	$ 8,017,663,000	$ 5,992,269,000
Agency MBS | Repurchase Agreement Borrowings
Repurchase Agreements
Fair Value of securities pledged as collateral under repurchase agreements	6,746,336,000	5,366,345,000
Balance of repurchase agreements	6,415,512,000	5,057,328,000
Non-Agency MBS | Repurchase Agreement Borrowings
Repurchase Agreements
Fair Value of securities pledged as collateral under repurchase agreements	2,502,540,000	1,329,625,000
Cash pledged under repurchase agreements		6,844,000
Non-Agency MBS acquired from consolidated VIEs	2,458,000,000	462,000,000
Balance of repurchase agreements	1,602,151,000	934,941,000
Non-Agency MBS | Linked Transactions
Repurchase Agreements
Linked repurchase agreements	193,000,000	567,300,000
Within 30 days: | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	5,835,230,000	3,986,428,000
Weighted Average Interest Rate (as a percent)	0.48%	0.61%
Over 30 days to 3 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	2,052,674,000	1,879,741,000
Weighted Average Interest Rate (as a percent)	0.75%	0.39%
Over 3 months to 6 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	111,559,000	96,100,000
Weighted Average Interest Rate (as a percent)	1.00%	0.48%
Over 6 months to 12 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	5,200,000	7,700,000
Weighted Average Interest Rate (as a percent)	3.15%	3.15%
Over 12 months to 24 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	13,000,000	12,300,000
Weighted Average Interest Rate (as a percent)	3.15%	3.15%
Over 24 months to 36 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements		10,000,000
Weighted Average Interest Rate (as a percent)		3.15%
Within 30 days | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	5,418,445,000
Weighted Average Interest Rate (as a percent)	0.40%
Over 30 days to 90 days | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	2,127,920,000
Weighted Average Interest Rate (as a percent)	0.78%
Over 90 days to 12 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	412,943,000
Weighted Average Interest Rate (as a percent)	1.34%
Over 12 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	58,355,000
Weighted Average Interest Rate (as a percent)	2.44%
Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	$ 8,017,663,000	$ 5,992,269,000
Weighted Average Interest Rate (as a percent)	0.56%	0.55%
Weighted average remaining term-to-interest rate reset of borrowings under repurchase agreements (in days)	30	34
Effective repricing period (in months)	10	12